1. Field of the Invention
The invention relates to scanning probe microscopes (SPMs) and, more particularly, relates to a SPM that facilitates the location and data acquisition from a small region of interest on the order of microns from a large sample and to a method of operating such an SPM
2. Description of the Related Art
Scanning probe microscopes (SPMs), such as the atomic force microscope (AFM), are devices which typically use a tip and low tip-sample interaction forces to characterize the surface of a sample down to atomic dimensions. Generally, SPMs include a probe having a tip that is introduced to a surface of a sample to detect changes in the characteristics of a sample. By providing relative scanning movement between the tip and the sample, characteristic data can be acquired over a particular region of the sample and a corresponding map of the sample can be generated.
The atomic force microscope (AFM) is a very popular type of SPM. The typical AFM employs a probe having a cantilever and a tip. A scanner generates relative motion between the probe and a sample while the probe-sample interaction is monitored. In this way, images or other measurements of the sample can be obtained. The scanner typically includes one or more actuators that usually generate motion in three orthogonal directions (XYZ). The probe is often coupled to an oscillating actuator or drive that is used to drive probe at or near a resonant frequency of cantilever. Alternative arrangements measure the deflection, torsion, or other motion of cantilever. A controller generates control signals to maintain either a relative constant interaction between the tip and sample or a constant deflection of the cantilever. Measurement involves controlling the scanner to move either the sample or the probe up and down relatively perpendicular to the surface of the sample under feedback. The scanner may be either a “sample scanner” that supports the sample or a “tip scanner” that support the probe. In any event, a translation stage may be provided for additionally translating the sample in at least X Andy and possibly Z to position the sample beneath the probe. The scanner is controlled to perform a scan operation by effecting relative probe-sample motion in an “x-y” plane that is at least generally parallel to the surface of the sample. Note that many samples have roughness, curvature and tilt that deviate from a flat plane, hence the use of the term “generally parallel.” The term “parallel” may also be used herein and should be construed to also mean “generally parallel.” The scan typically takes the form of a raster scan in which data is taken along lines in the X direction that are closely spaced in the Y direction. The maximum length of the lines in the X direction is known as the “scan range.” In this way, the data associated with this vertical motion can be stored and then used to construct an image of the sample surface corresponding to the sample characteristic being measured, e.g., surface topography.
Many SPMs are incorporated into an SPM assembly that additionally has an integrated optical microscope including a high resolution camera to facilitate navigation of the sample with respect to the SPM. The camera and associated optics (simply referred to as a camera herein for the sake of simplicity) typically have a relatively narrow field of view on the order of 1 to 2.5 mm. The camera typically provides a video image that may itself be recorded and manipulated.
Regardless of their mode of operation, AFMs can obtain resolution down to the atomic level on a wide variety of insulating or conductive surfaces in air, liquid or vacuum by using piezoelectric scanners, optical lever deflection detectors, and very small cantilevers fabricated using photolithographic techniques. Because of their resolution and versatility, AFMs are important measurement devices in many diverse fields ranging from semiconductor manufacturing to biological research.
The most broadly adopted commercial SPMs usually require a total scan time of several minutes to cover an area of several square microns at medium-high resolution (e.g. 512×512 pixels), low tracking force, and high image quality. At even higher data densities, such as 1024×1024 and above, data density is sufficiently high permit one to zoom in on captured data and still have enough data density to be useful. Extremely high data density of 5,000×5,000 pixels produces images of exceptional quality, but such scans take 83 minutes at 1 Hz scan speeds. In general, the practical limit of SPM scan speed is a result of the maximum speed at which the SPM can be scanned while maintaining a tracking force that is low enough not to damage the tip and/or sample or to at least limit the damage to the tip and/or sample to acceptable levels.
However, recent work in high-speed SPM has been performed by a number of groups. This work has culminated in the assignee's development of an AFM that can scan large ranges very rapidly with high resolution. Scan speeds in excess of 10 Hz are possible while still maintaining extremely high resolution and preserving tip integrity.
One drawback of existing AFM and other SPM designs, including the recently developed high-speed, high resolution AFM developed by the assignee, is that locating a feature of interest on a sample using the camera of an AFM assembly and acquiring usable data from that feature using the AFM probe can be a very time-consuming process requiring a high level of skill on the part of the AFM operator. This drawback is due in part to the fact that the optics employed in current AFMs, while having a relatively high magnification range, necessarily have only a narrow field of view on the single mm scale. This problem is exasperated by the fact that locating a region of interest within that field of view using the AFM proper can be a difficult and/or time consuming process. In addition, the AFM microscope tends to obscure the sample, making visual observation of the sample location under the AFM difficult.
Specifically, referring to the flowchart of FIG. 1, the typical process 110 for acquiring data from a sample proceeds from START in Block 112 to Block 114, where the user places the sample on the support or “chuck” of the AFM assembly that is itself supported on the translation stage. The sample may be quite large—on the order of more than 150 mm in diameter and even substantially larger. The feature of interest on that sample typically will be found in an “area of interest” of no more than a few millimeters, and the feature of interest itself often will be in a “region of interest” having a size of the nanometer size range, typically between 100 nm to 10 microns and, thus, is far too small to be seen with the naked eye.
Next, in Block 116, the user manually operates the translation stage to attempt to position the area of interest within the field of view of the AFM assembly's camera assembly's optics while manually peering between the camera and the underlying chuck. The user then views the video image from the camera in Block 118 and determines in Block 120 whether the area of interest is within the AFM assembly's optical field of view. The area of interest might not be visible at this time, even if it is nominally within the optical field of view, if the camera is out of focus and/or the instrument's illumination is set incorrectly. The user thus may have to adjust the focus of the camera and/or alter the illumination to even determine whether or not the area of interest is within the camera's field of view. The AFM probe usually is mounted so as to move up and down with the camera. If the sample has no features visible in the video image, the user may move the camera too close to the sample and ram it into the sample surface while attempting to focus the camera, resulting in potential damage to the probe. Hence, once again, considerable skill and some level of luck are required for this step.
After these adjustments, if the area of interest is not found in Block 120, the user has to move the translation stage to a new position and repeat the operations of Blocks 116-120 until the area of interest is located in the camera's field of view in Block 120. (For small samples, the area itself may be considered the region of interest). The area of interest may be found, for example, by noting a change in contrast between the area of interest and the surrounding portions of the sample surface.
Next, in Block 122, the user manually moves the translation stage while viewing the video image to center the area of interest within the AFM assembly camera's optical field of view.
Next, in Block 124, the user engages the AFM's probe and operates the AFM it to scan the feature of interest while acquiring high density data of on the order of 512×512 pixels or above. This is no small feat given the fact that the time required to scan the entire area of view of an AFM typically prohibits scanning the entire area using a high data density scan of on the order of 1024×1024 pixels or higher and then acquiring data from the feature of interest from the scanned data. Such an operation typically would take nearly an hour using current AFM technology. It would take considerably longer, in fact in excess of 83 minutes, if an extremely high density scan of on the order of 5000×5000 pixels were performed.
The most commonly employed alternative to this procedure is a so-called “pan and zoom” technique. In this technique, the user scans a relatively small area of, e.g., 10×10 microns within the AFM's scan range and analyzes the data in that area to determine whether the feature of interest has been captured. If not, the user repeats this process in randomly distributed or a methodically determined pattern of scan areas within the area of interest until the region of interest is located. Some users perform each 10×10 micron scan at a mid-density level of, e.g., 256×256 pixels, taking about 4 minutes, until the feature of interest is found. They then perform a higher density scan of the small area that contains the region of interest after the feature of interest is located.
A third alternative that essentially is a combination of the first two alternatives is to first capture a large, very low density (on the order 128×128 pixel) survey scan of the entire scannable area to attempt to locate the feature of interest and, upon locating that feature, zooming into the region of interest containing the feature of interest and capturing a smaller, high data density image of the region containing that feature. This alternative may be considerably faster than the first alternative but risks missing the feature of interest entirely if the data density of the survey scan is too low to find that feature.
Regardless of the technique used to acquire the high data density scan of the feature of interest, the resulting image data of that feature is captured and analyzed in Block 126, and the routine returns to END in Block 128.
Depending upon factors such as the skill level of the operator, the success the operator has in locating the area of interest within the sample, the region of interest in that area, and the technique employed to capture data from the area of interest, the above-described process can take anywhere from many minutes to over an hour from the initiation of the process to the capture and analysis of the high density image data of the feature of interest. It can fail altogether if the operator is insufficiently skilled. In addition, scanner drift and/or changes in sample feature attributes, such as location, size, and shape, occurring between the time that the feature of interest is located and the time that the data concerning that feature is acquired and analyzed can lead to acquisition of out-of-date data and image distortion.
Hence, the need exists to provide a SPM system and process that are capable of rapidly locating a region of interest on a sample and obtaining and analyzing data concerning that region, preferably within a matter of minutes.
The need additionally exists to provide a SPM system and process that permit a relatively unskilled operator to capture and analyze data concerning a small region of interest on a sample surface while minimizing or eliminating risk of damage to the SPM.